1. Field of the Invention
The present invention relates to an optical disk and an optical disk apparatus wherein light is condensed at a very small spot to perform recording or playback of information, and also to a method of correcting an aberration appearing on a very small spot upon recording or playback of information using an optical disk of the type just mentioned.
2. Description of the Related Art
An optical disk is known as an information recording medium which allows high density recording without contacting with the same, and a compact disk (CD), a digital versatile or video disk (DVD) and so forth have been put into practical use. In high density recording, wherein an optical disk of the type described is used, in order to achieve an increase in the capacity and increase in the density, the numerical aperture of a condenser of an optical system to be used for formation of a very small spot is increasing. For example, where the numerical aperture for a CD in the past was 0.45, the numerical aperture for a DVD which allows higher density recording is 0.6. It is possible that a condenser of a further high numerical aperture of 0.8 or more will be used in the future.
Recording or playback of information onto or from an optical disk is performed usually by forming a very fine spot on an information recording layer of the optical disk. FIG. 15 schematically illustrates a manner in which a very small spot is formed on an information recording layer of a conventional optical disk.
Referring to FIG. 15, an optical disk 100 is of the type generally placed in practical use and includes a protective transparent substrate 101 which covers a surface of an information recording layer 102. A condenser 103 is a component of an optical system of a recording-playback section of a known optical disk apparatus, and a laser beam for information playback or for information recording is condensed by the condenser 103 so that a very small spot is formed on the surface of the information recording layer 102 of the optical disk 100. The optical disk 100 is structured such that the information recording layer 102 having a very small structure of less than micrometer order is protected by the transparent substrate 101 so that reliability in recording or playback of information can be assured.
In a configuration where a laser beam condensed by the condenser 103 passes through the transparent substrate 101 and forms a very small spot on the surface of the information recording layer 102 as described above, if the numerical aperture of the condenser 103 is increased, then the influence of an aberration which appears with the spot when the thickness of the transparent substrate 101 is displaced from a designed value increases, and it becomes difficult to form a very small spot. If the numerical aperture becomes, for example, 0.85, then the tolerance of the displacement of the thickness of the transparent substrate becomes less than several xcexcm.
An influence of a spherical aberration upon a very small spot when the displacement of the thickness of the transparent substrate is within its tolerance and when the displacement is outside its tolerance is described.
FIG. 16 illustrates a variation of the spot intensity distribution with respect to a variation of the focus displacement where the displacement of the thickness of the transparent substrate is within its tolerance (the thickness of the transparent substrate is substantially within a designed range (prescribed range)). In this instance, the beam diameter exhibits a substantially symmetrical variation before and after a focused point. Here, the focused point signifies a position in which the optical system including the condenser is focused on the object side (optical disk side) and corresponds to a focused point of a camera.
However, if the substrate thickness exceeds the tolerance, then a spherical aberration which provides concentric waveform variations appears. FIG. 17 illustrates a variation of the spot intensity variation with respect to the variation of the focus displacement where the displacement of the thickness of the transparent substrate exceeds the tolerance. In this instance, the spot intensity distribution with respect to the variation of the focus displacement varies asymmetrically before and after the focused point and exhibits an irregular variation such that it has an increased side lobe or has an increased beam diameter at a certain portion thereof. In such a condition, it is difficult to obtain a good playback signal.
In such a case that the displacement of the thickness of the transparent substrate exceeds its tolerance, it is demanded to detect an aberration amount caused by a displacement in thickness of a transparent substrate by some method and correct an influence of the aberration.
Such an aberration detection apparatus as disclosed in Japanese Patent Laid-Open No. 2000-20999 has been proposed. FIG. 18 is a schematic view showing a general configuration of the aberration detection apparatus disclosed in the document just mentioned.
Referring to FIG. 18, the aberration detection apparatus incorporates an interference optical system in an optical system for detecting reflected light from an optical disk and detects an aberration amount from a variation of the intensity distribution caused by interference. The aberration detection apparatus includes an interference plate 200, a convergent lens 201, and a split light detector 202. Reflected light from an optical disk, or light condensed by a condenser (not shown) and reflected from the optical disk, passes through the condenser again, and part of the reflected light is split and enters the interference plate 200, In the interference plate 200, the incoming reflected light is split, and the split light components are overlapped with each other in a state wherein they are spatially displaced from each other a little. The overlapped light is introduced into the split light detector 202 by the convergent lens 201. If an aberration occurs, then the interference condition changes and the aberration can be observed as a variation of the light intensity distribution by the split-light detector 202.
When the numerical aperture of a condenser for forming a very small spot is increased in order to achieve further increase of the capacity and the density of an optical disk, if the displacement of the thickness of the transparent substrate for protecting the information recording layer of the optical disk exceeds its tolerance, then it is necessary to detect the aberration amount caused by the displacement of the thickness of the transparent substrate by some method and correct the influence of the aberration.
Although the aberration detection apparatus disclosed in Japanese Patent Laid-Open No. 2000-20999 mentioned above can solve the subject described above, since it need include a new optical system for detecting an aberration in an optical system of an optical disk apparatus, this gives rise to a new problem that it increases the scale of the apparatus and increases the cost.
It is an object of the present invention to provide an optical disk, an aberration correction method and an optical disk apparatus wherein a spherical aberration caused by a displacement of the thickness of a transparent substrate of the optical disk can be corrected without giving rise to an increase in scale of the apparatus or an increase of the cost.
In order to attain the object described above, according to an aspect of the present invention, there is provided an optical disk, comprising an information recording layer having a recording face on which a very small spot for recording or playing back information is formed, and a transparent substrate for covering the recording face of the information recording layer, the recording face of the information recording layer having a particular region in which a particular pattern from which a spherical aberration of the very small spot which appears in accordance with a displacement of the thickness of the transparent substrate from a prescribed value can be detected is formed.
With the optical disk, since the particular pattern from which a spherical aberration of a very small spot which appears in accordance with a displacement of the thickness of the transparent substrate from a prescribed value (the displacement is hereinafter referred to as substrate thickness displacement) can be detected is formed in the particular region of the recording face, a spherical aberration can be detected by playing back the particular pattern. Since a conventional optical system for playback can be used for the playback of the particular pattern, there is no necessity to provide a new optical system in order to detect the spherical aberration.
The particular region may be part of a lead-in region formed in an innermost circumference region of the disk. With the optical disk, a spherical aberration caused by a substrate thickness displacement can be detected in an initial stage of playback of the disk.
The particular region may be formed at a plurality of locations of different radial positions of the disk. In this instance, the particular region may be formed at each of part of a lead-in region formed in an innermost circumference region of the disk and part of a lead-out region formed in an outermost circumference region of the disk.
Where the particular region is formed at a plurality of locations of different radial positions of the disk, aberration correction can be performed with regard to the regions at the different radial positions of the disk. Consequently, where the thickness of the substrate exhibits a variation in a radial direction of the disk (radius dependency) like a transparent substrate which is produced, for example, by injection molding, aberration correction can be performed in accordance with the variation of the substrate thickness in the radial direction of the disk. In this instance, where the particular region is formed at each of part of the lead-in region formed in an innermost circumference region of the disk and part of the lead-out region formed in an outermost circumference region of the disk, by playing back the particular pattern in the particular regions, a correction amount for the spherical aberration on the innermost circumference side and a correction amount for the spherical aberration on the outermost circumference side of the optical disk are obtained. Thus, a correction amount for a spherical aberration in a portion of the optical disk in which data pits are recorded between the lead-in region and the lead-out region can be calculated readily through interpolation processing from the thus obtained correction amounts.
The particular pattern may include a plurality of pit trains having different periods from each other and disposed alternately. In this instance, the particular pattern may include a minimum period and a maximum period of coded data to be recorded on the recording face of the information recording layer. Further, a plurality of pit trains having different periods from each other may be disposed alternately over an entire circumference in the same track.
Where the particular pattern includes a plurality of pit trains having different periods from each other and disposed alternately, playback signals which have signal amplitudes different depending upon the periods of the pit trains are obtained, and if an amplitude variation of each of the playback signals of the pit trains with respect to the focus displacement amount is determined, then the amplitude variations exhibit a considerable difference in focus displacement amount (focus offset amount) with which a maximum amplitude is provided. The focus offset amount corresponds to a spherical aberration of a very small spot caused by the thickness displacement of the transparent substrate. Consequently, the spherical aberration can be corrected by such correction as to minimize the difference between the focus off set amounts of the pit trains. Further, where the particular pattern includes a minimum period and a maximum period of coded data to be recorded on the recording face of the information recording layer, aberration correction for all of the periods of recorded pits of coded data can be performed through interpolation processing. Furthermore, where a plurality of pit trains having different periods from each other are disposed alternately over an entire circumference in the same track, a spherical aberration can be corrected in accordance with the thickness variation of the transparent substrate in a circumferential direction.
The particular pattern may include pits or spaces between pits of a first length and pits or spaces between pits of a second length disposed alternately in a fixed period. With the optical disk, the ratio between the amplitude of the playback signal from the pits or the spaces between pits of the first length and the amplitude of the playback signal from the pits or the spaces between pits of the second length varies in accordance with the spherical aberration caused by the thickness displacement of the transparent substrate. Consequently, the spherical aberration can be corrected by setting the amplitude ratio of the playback signals of the pits or the spaces so as to fall within a predetermined range.
According to another aspect of the present invention, there is provided an aberration correction method, comprising a first step of forming, in a particular region of a recording face of an information recording layer of an optical disk wherein the recording face of the information recording layer on which a very small spot for recording or playing back information is formed is covered with a transparent substrate, a particular pattern wherein a plurality of pit trains having periods different from each other are disposed alternately, a second step of moving a condenser for forming the very small spot along an optical axis of the condenser to play back the particular pattern to determine amplitude variations of playback signals of the pit trains with respect to a focus error amount, a third step of determining focus offset amounts each of which provides a maximum amplitude from the amplitude variations of the playback signals of the pit trains determined in the second step, and a fourth step of adjusting the angle with which a light beam incoming to the condenser diverges or converges so that the difference between the focus offset amounts of the pit trains determined in the third step may be minimized to correct the spherical aberration of the very small spot which appears in accordance with a displacement of the thickness value of the transparent substrate from a prescribed value.
According to a further aspect of the present invention, there is provided an aberration correction method, comprising a first step of forming, in a particular region of a recording face of an information recording layer of an optical disk wherein the recording face of the information recording layer on which a very small spot for recording or playing back information is formed is covered with a transparent substrate, a particular pattern wherein pits or spaces between pit shaving a first length and pits or spaces between pits having a second length are disposed alternately in a fixed period, and a second step of adjusting the angle with which a light beam incoming to the condenser diverges or converges so that the ratio between the amplitude of a playback signal from the pits or the spaces between pits of the first length and the amplitude of another playback signal from the pits or the spaces between pits of the second length may have a predetermined value to correct the spherical aberration of the very small spot which appears in accordance with a displacement of the thickness value of the transparent substrate from a prescribed value.
Both of the aberration correction methods may further comprise a step of correcting a frequency characteristic of a signal output of a photo-detector which detects reflected light from the pits formed on the recording face of the information recording layer so as to be a predetermined frequency characteristic to correct the spherical aberration of the very small spot occurring in accordance with the displacement of the thickness of the transparent substrate from the prescribed value.
According to a still further aspect of the present invention, there is provided an optical disk apparatus, comprising a condenser movable along an optical axis thereof for forming a very small spot for recording or playing back information on a recording face of an information recording layer of an optical disk wherein the recording face of the information recording layer is covered with a transparent substrate, an aberration correction element for adjusting the angle with which a light beam incoming to the condenser diverges or converges, a photo-detector for detecting reflected light from a pit formed on the recording face of the information recording layer, and control means for controlling the adjustment of the diverging or converging angle of the incoming light beam by the aberration correction element so that the amplitude of a playback signal, outputted from the photo-detector, of a particular pattern formed in a particular region of the recording face of the information recording layer from which a spherical aberration of the very small spot occurring in accordance with a displacement of the thickness of the transparent substrate from a prescribed value can be detected may be a predetermined amplitude.
The control means may include aberration detection means for moving the condenser along the optical axis of the same to obtain, from the output of the photo-detector, a playback signal of the particular pattern formed in the particular region of the recording face of the information recording layer and including a plurality of pit trains having periods different from each other and disposed alternately, determining, from the obtained playback signal, amplitude variations of the playback signals of the pit trains with respect to a focus error amount and determining focus offset amounts each of which provides a maximum amplitude from the determined amplitude variations of the playback signals of the pit trains, and incoming angle control means for controlling the adjustment of the angle with which the incoming light beam diverges or converges by the aberration correction element so that the difference between the focus offset amounts of the pit trains determined in by the aberration detection means may be minimized.
Alternatively, the control means may include aberration detection means for determining, from the playback signal, outputted from the photo-detector, of the particular pattern formed in the particular region of the recording face of the information recording layer and including pits or spaces between pits of a first length and pits or spaces between pits of a second length disposed alternately in a fixed period, a ratio between the amplitude of the playback signal from the pits or the spaces between pits of the first length and the amplitude of the playback signal from the pits or the spaces between pits of the second length, and incoming angle control means for controlling the adjustment of the diverging or converging angle of the light beam by the aberration correction element so that the amplitude ratio determined by the aberration detection means may have a predetermined value.
The optical disk apparatus may further comprise a variable frequency characteristic amplifier for correcting a frequency characteristic of the signal output of the photo-detector so as to be a predetermined frequency characteristic.
The optical disk apparatus may further comprise storage means for storing adjustment values of the diverging or converging angle of the incoming light beam by the aberration correction element set so that the amplitude of the playback signal of the particular pattern may be a predetermined amplitude, the control means calculating, upon playback of a pit train other than the particular pattern formed on the recording face of the information recording layer, an adjustment value for the pit train through interpolation from the adjustment values stored in the storage means in accordance with a period of the pit train and a radial position of the disk and performing the adjustment of the diverging or converging angle of the incoming light beam by the aberration correction element with the adjustment value calculated through the interpolation.
With the aberration correction methods and the optical disk apparatus, the spherical aberration of a very small spot caused by the thickness displacement of the transparent substrate can be corrected in accordance with the action of the optical disk of the present invention described above.
According to a yet further aspect of the present invention, there is provided an optical disk apparatus, comprising a photo-detector for detecting reflected light from a pit formed on a recording face of an information recording layer of an optical disk wherein the recording face of the information recording layer is covered with a transparent substrate, and a variable frequency characteristic amplifier for correcting a frequency characteristic of a signal output of the photo-detector so as to be a predetermined frequency characteristic.
With the optical disk apparatus, since the signal amplitude played back from an optical disk exhibits, where the spherical aberration is not very great, such a variation that a high frequency side portion of the signal is emphasized as hereinafter described in detail, the variation of the frequency characteristic can be corrected by the variable frequency characteristic amplifier so that it may become a predetermined frequency characteristic which is an original frequency characteristic free from any aberration.
In summary, according to the present invention, only by forming a particular pattern in a particular region of an optical disk, a spherical aberration caused by a displacement of the substrate thickness can be detected and corrected without providing a special optical system for detecting the aberration on the optical head side, and therefore, an optical disk and an optical disk apparatus which can be designed as products without giving rise to an increase in size or an increase in cost of the apparatus can be anticipated.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference symbols.